The invention relates to novel polymers and to a process for preparing the same. It also relates to products, and in particular food products, comprising the polymer. The invention also particularly relates to edible films formed from the polymer.
Various edible films are known in the art. For example, Motoki et al have disclosed a milk-protein based film comprising xcex1s-1-casein cross-linked using transglutaminase (Agric Biol Chem 51(4): 993-996, (1987)). Caseinate films are also known, for example from the work of Avena-Bustillos and Krochta (J Food Sci 58(4): 904-907 (1993)). Whey protein-based edible films are also known, for example, from the work of McHugh et al (J Food Sci in press, (1994)). However, these prior art films suffer from the disadvantages that they are either readily soluble or that they tend to be lacking in tensile strength.
An insoluble edible film with tensile strength would be desirable for such applications as separating liquids from solids in the same product. For instance, such a film could be used to encapsulate liquids or slow release additives (such as bread improvers) which cannot be incorporated into soluble films since they disperse instantly.
It is the object of the present invention to provide a polymer which is suitable for use as an edible film and which will go some way towards overcoming the disadvantages of the prior art, or at least to offer the public a useful choice.
In a first aspect the present invention provides a polymer comprising acid casein or a non-toxic soluble salt thereof and high-methoxyl pectin cross-linked into a 3-dimensional network.
In a second aspect the present invention provides a polymer comprising acid casein or a non-toxic soluble salt thereof and high-methoxyl pectin, wherein the polymer has been prepared by reacting the acid casein or non-toxic soluble salt thereof and the high-methoxyl pectin under alkaline conditions.
In still a further aspect the invention provides a food product comprising a polymer as defined above.
In a further aspect, the invention provides an edible film comprising a polymer as defined above.
The invention also provides a food product which includes an edible film as defined above, either internally or as a coating on all or part of an external surface of the food product.
In a preferred embodiment of the invention, the polymer further comprises an edible plasticizer such as glycerol.
In yet a further aspect, the present invention provides a process for preparing a polymer comprising acid casein or a non-toxic soluble salt thereof and high-methoxyl pectin, the method comprising the steps of:
(a) forming a dispersion comprising acid casein or a non-toxic soluble salt thereof and high-methoxyl pectin, in a non-toxic alkaline environment,
(b) heating the disperion for a sufficient time and at a sufficient temperature for reaction between the acid casein or non-toxic soluble salt thereof and the high-methoxyl pectin to take place;
(c) cooling the resulting reaction mixture; and
(d) separating off the resulting aqueous phase containing the polymer.
Preferably, the method further includes the step of:
(e) heating the aqueous phase for a sufficient time and at a sufficient temperature to set the polymer.
In more preferred embodiments, the dispersion formed in step (a) includes an edible plasticizer such as glycerol.
In yet further aspects the invention provides a polymer prepared by a method as defined above, a food product comprising a polymer prepared by a method as defined above, and an edible film comprising a polymer as defined above.
While the present invention is broadly defined above, it will be appreciated by those persons skilled in the art that it is not limited thereto but that it also includes embodiments of which the following description provides examples. In particular, the various aspects of the invention will be better understood with reference to the more detailed description provided below.
In its primary aspect, the invention provides a novel polymer. This polymer is formed from acid casein, or a non-toxic soluble salt thereof, and high methoxyl pectin.
The polymers of the invention can be prepared by reacting acid casein or a non-toxic soluble salt thereof with high-methoxyl pectin, under alkaline conditions. Without wishing to limit the scope of the invention in any way, it is believed that the Rxe2x80x2-COOCH3 group which is present in high-methoxyl pectin condenses with free Rxe2x80x3-NH2 groups, which are present as side chains on acid casein or its salts in the presence of base. Thus, it is believed the product of the reaction is an amide which links the ester with the amine, forming Rxe2x80x2-CONH-Rxe2x80x3, thus liking acid casein with high-methoxyl pectin into a 3-dimensional network.
In its preferred forms, the polymers of the invention include an edible (and therefore non-toxic) plasticiser. This plasticiser is included to enhance the flexibility of the resultant polymer and to prevent it becoming brittle.
The presently preferred plasticiser is glycerol although any non-toxic edible plasticiser can be used.
Preparation of Polymer
The preferred methods of preparing the polymers of the invention will now be described in more detail. As an initial step of the preferred method of preparation, a dispersion is formed comprising acid casein or a non-toxic soluble salt thereof and high-methoxyl pectin, in an alkaline environment.
It will be appreciated that either acid casein or a non-toxic soluble salt of acid casein (such as sodium caseinate) may be used to prepare the polymers of the invention. If acid casein itself it used, it will of course be converted to the caseinate form in the alkaline reaction environment.
It is preferred that the acid casein or non-toxic soluble salt thereof is present in an amount of about 8-12% by weight of the dispersion, and more particularly about 10% by weight. In a particularly preferred embodiment, acid casein is present in an amount of 10.29%.
xe2x80x9cHigh-methoxyl pectinxe2x80x9d is defined herein as alpha 1, 4 linked polygalactironic acid intermittently esterified at the C6 position with methyl groups, and in which the proportion of C6 esters is not less than about 32%.
The high-methoxyl pectin is preferably present in an amount of about 1 to 3 % by weight of the dispersion, and more preferably about 1.7% by weight. In a particularly preferred embodiment, the high-methoxyl pectin is present in an amount of 1.71% by weight. It is also preferred that the degree of methylation of the high-methoxyl pectin exceeds about 32%. In a particularly preferred embodiment, the degree of methylation is about 60%. It is also preferred that the ratio acid casein or non-toxic soluble salt thereof: high-methoxyl pectin is in the range of about 5.1:1 to 6:1, more preferably 6:1, in order to obtain a polymer having the most desirable properties in terms of insolubility.
In some commercially available preparations of high-methoxyl pectin, there is some residual low molecular weight material. The presence of such material in the polymers of the present invention can render them soluble. Accordingly, it is generally preferred that, prior to use, such low molecular weight material is removed from the high methoxyl pectin. This can be achieved for example by dialysis, or by suspending the high-methoxyl pectin in a suitable solvent, such as ethanol, which will dissolve the low molecular weight material.
The alkaline reaction environment is preferably produced by a non-toxic, non-effervescent base in water. The pH is required to be alkaline in order for the reaction between the acid casein and high-methoxyl pectin to take placed. If however, the pH of the dispersion varies much above about 8.5 to about 9, the resulting polymers may be too alkaline to be edible. In a preferred embodiment of the invention, the base is sodium carbonate, and is preferably present in an amount sufficient to adjust the pH of the dispersion to about 8.5 to 9. Thus, the sodium carbonate is preferably present in an amount of about 1-2% by weight of the dispersion, and, in a particularly preferred embodiment, 1.6% by weight.
Where as is preferred the polymer is to contain an edible plasticiser, the plasticiser is included in the disperion. The edible plasticiser, usually glycerol, is present in the dispersion in an amount of about 4% to 8% by weight of the dispersion, and, in a particularly preferred embodiment, 6% by weight.
The temperature of the water during formation of the dispersion is preferably in the range of about 60xc2x0 C. to 80xc2x0 C., more preferably 65xc2x0 C. to 75xc2x0 C., and, in a particularly preferred embodiment, 70xc2x0 C.
When the acid casein, high-methoxyl pectin, sodium carbonate and glycerol are dispersed in the water, the mixture is heated under pressure, conveniently by autoclaving, and preferably to a temperature of about 120xc2x0 C. (more preferably 121xc2x0 C.), for about 3 to 7 minutes, more preferably 5 minutes. A relatively high temperature such as about 120xc2x0 C. is desirable in assisting the reaction between acid casein and high-methoxyl pectin (believed to be condensation) to take place. However, if the temperature is raised much above 120xc2x0 C., the mixture may brown, which is undesirable.
Following autoclaving, the reaction mixture is cooled, usually to room temperature. The unreacted base will usually precipitate out. The proportions of reactants in the particularly preferred embodiment described in the Example below, have been found to be suitable for achieving this precipitation, thereby avoiding crystals or flecks of base being retained in the polymer. The aqueous phase containing the polymer is then separated out. In some preferred embodiments of the invention, which will be described below, the aqueous phase is poured or sprayed onto a surface to form a film.
The polymer is routinely set by heating. A temperature of about 140xc2x0 C. to 160xc2x0 C., and a heating time of about 2 to 4 minutes have been found to be particularly suitable for setting the polymers of the invention. Where, as in a preferred embodiment, the polymer is to be used to form a film, the films are heated at 150xc2x0 C. for 3 minutes. The heating is desirable in order to set the film quickly. If the film is not set quickly it will continue to flow and can become too thin to be useful.